Final answer:
Potassium has an atomic number of 19 and a rounded mass number of 39. Subtraction of the atomic number from the mass number (39 - 19) reveals that potassium has approximately 20 neutrons.
Step-by-step explanation:
Potassium, with an atomic number of 19, possesses 19 protons, which also corresponds to its number of electrons. Its average atomic mass is given as 39.10 atomic mass units (amu). To find the number of neutrons in potassium, we subtract the atomic number from the rounded atomic mass, which is the nearest whole number to the average atomic mass and typically referred to as the mass number in this context.
The mass number of potassium is 39 when we round off 39.10 amu to the nearest whole number. The calculation therefore is: number of neutrons = mass number - atomic number = 39 - 19 = 20. Thus, a potassium atom has approximately 20 neutrons.
To provide an example related to potassium's isotopes, K-41 is a naturally occurring isotope of potassium with a mass number of 41, which means K-41 has 41 - 19 = 22 neutrons, differing from the normal potassium atom discussed above.